This weekend I did something I don't really practice, that's leaving the Roadster plugged in from Friday evening to Monday morning topped off on a standard mode charge. I typically leave the pack sit at 50-60% SOC over the weekend since that's the SOC the lithiums like best (and 26F or below temp). Every morning I charge the roadster so it has 1/2 hour to balance and typically the pack balances out in 15-20 mins. The balancing grid of diags is all 0's if fully balanced and shows 1's in the brick/sheet matrix that need to be brought up and balanced. I use to think was the true tale of balancing, but I found out its not! Its just a really close rule. When I tapped into my Zero electric motorcycle's com port (hack) you can get all types of cool data and tweak the bike a bit. The nice thing that, I wish this was public accessible, are the balancing values. In the Zero instead of a 0 or a 1 it shows how far off each cell is in Volts from one another. In this fluke of an experiment, my CAC stayed the same, around 155.13 but my ideal standard mode range had increased by 3-4 miles on my Tuesday morning's charge. All after sitting at 80-84% SOC for the weekend. Every charge now hits 187 and that's after sitting for 2-3 hours so its a real number. Before this experiment, I was charging up to 184 ideal miles consistently and also didn't seem to stick as well as the 187 after I drive off you can see the miles drop fast sometimes in 2-5 miles of real tarmac. So now it appears that if I want to keep the pack truly balanced to its optimal peak that I need to let it sit for a weekend in std. mode every so often. I'll see how long this 187 std charge range will hold true. I'm guessing every 4-6 months doing this to keep things optimum.

The Roadster keeps its battery at 85% in Standard mode, which is better than 100%, but dropping to <75% is easier on the pack. One downside to doing this (besides less range) is the pack gets fewer opportunities to balance. Doing an occasional Standard or Range mode charge helps to keep the cells in balance while doing little damage, as long as the pack isn't kept at high SOC.

I've seen in other references as 50% where motorola states 30-40%. I think long term storage mode of the Roadster will allow it to trickle down to 30% for is floor SOC. I believe Tesla did their homework and that leaving it plugged in on Standard mode is fine. Best case (which will most likely buy you more cycles of charging) is closer to 50%. However I feel it is something that's not that much more of work to manage. Here's a table, would like to see the data they used and plot it against the SOC from 0-100 and see how 50% and 80-85% compare. However if its more work for someone you're not really hurting the pack leaving it plugged in all the time in Std. Mode. Sometimes its better, less chance of draining to 0% and bricking!

Note this is a table for batteries stored at that SOC and temp (trying to understand the 3 months in the table vs. the 1 year on the side comment). So for me I try to match this table by always running a cool-down dropping the pack below 26C and keeping the pack close to 50% charge. Times adds up as well as average temps the pack's been at. Would be nice if the Roadster could track like a trip odometer the average temp and SOC of the pack.

" - Batteries should be stored at the optimum storage state of charge which is between 30% and 40%.
- Avoid storing battery packs at either full state of charge low state of charge. Batteries stored at full state of charge will age faster and batteries stored at low state of charge can cause cell damage and activation of low cell lockout level which can permanently disable the battery.
- Storage longer than 6 months is not recommended for Lithium Ion battery packs. Where conditions require longer term storage then a regular scheduled maintenance plan should be implemented to ensure that the battery packs are kept at the optimum state of charge. The maintenance plan should include periodic testing of the battery stated of charger and brief charging where required to maintain optimum state of charge."

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Thanks djp, I think it was you that made me question myself earlier on whether the Roadster was still fully balanced when it was reading all 0's and with the insight from the Zero battery / balancing granularity and this experiment it made sense and appeared to prove true. I'll report back in 5-6 months after my next weekend balancing cycle is due.

The main reason Tesla says that is so you don't completely forget about the car for a few months and accidentally brick the battery.

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Zero Motorcycles recommend the same thing, this question popped up in the electric motorcycle forum as well. The reason they recommend this is that they had a bug in one model (their X version) of the bike where the BMB would take away voltage while below 50% SOC draining one back of the pack to death just to balance the other bank of the pack. The other issue / bug was that there was a parasitic drain that would kill the battery if left unplugged over a month.

Best way to ensure the bikes don't brick and save customer support calls is to say "Always keep it plugged in". However keeping it plugged in means you're always at 100% SOC which is not the best place to be. Its trickier to play this 50% SOC game to better care for your pack when your pack has a low kilowatt hour capacity, for example the Nissan Leaf and my 2010 Zero DS which has only a 4.4kwh pack. The way I handle it is that if I know I won't be using the bike for a few days I'll keep the battery pack where its at as long as its above 30%. Luckily I don't have the parasitic drain. Also if I'm concerned I have an easy T-Handle I can pull that breaks the + and - of the pack (anderson connector disconnect). That way nothing is sipping away my energy.

In the summer I do the same as you and run between 35% and 50% SOC with occasional Standard or Range charges when I need them. We'll see in 10 years if it makes a difference. :smile:

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Right, its going to be a toss up and we may never know. I have a larger risk since my 12 aux battery components are tucked away inside of my ESS since it uses sheets 1 & 2. My original pack was replaced for that particular failure, so I guess my chances are better now! You have an even better chance since your ESS is simpler in design! Jealous. But then again I never have to replace a 12v battery so less maintenance for me. :tongue: But on the bright side I think we're at the point where warranties have expired, knowledge is building about dropping and caring for these packs. So chances are even if there was an internal failure, the pack could be dropped inexpensively and serviced with a skilled technician or pioneer with insight into getting the job done safely.